As for the rest, I will argue that using GrapheneOS would be far easier than something like Linux or whatever you may be thinking of.
It seems your threat model is high enough I don’t see dismissing this option being very justifiable on this.
And I just saw the camera and video thing, if I may I suggest NitroPhone:
They have the option to remove all the Sensors, camera and microphone.
But this is a permanent thing, not a kill switch, I doubt fairphone has a kill switch anywho though.
They also pre-install GrapheneOS for you.